1. Field of the Invention
The present invention relates to a solid-state image-sensing device, and more particularly to a solid-state image-sensing device of which each pixel is provided with an amplifying function.
2. Description of the Prior Art
Two-dimensional solid-state image-sensing devices are used in various applications. A two-dimensional solid-state image-sensing device has a large number of pixels arranged in a matrix (an array consisting of a plurality of rows and columns), and each of those pixels includes a photoelectric conversion element (photosensitive element) such as a photodiode and a means for transferring the photoelectric charge generated in it to an output signal line. Such solid-state image-sensing devices are classified roughly into a CCD type and a MOS type, according to the type of the means they use to read out (extract) the photoelectric charges generated in photoelectric conversion elements. CCD-type solid-state image-sensing devices transfer the photoelectric charges while accumulating them in potential wells, and have the disadvantage of narrow dynamic ranges. On the other hand, MOS-type solid-state image-sensing devices directly read out the electric charges accumulated in the pn-junction capacitance of photodiodes through MOS transistors.
FIG. 14 shows a conventionally known configuration of the latter type of solid-state image-sensing device. The solid-state image-sensing device shown in FIG. 14 includes a vertical scanning circuit 100 that performs scanning along the columns (in the vertical direction) by feeding, for one column after another, a signal φVRn and a signal φVSn to one of a plurality of reset lines 102 and one of a plurality of vertical selection lines 103, respectively, and a horizontal scanning circuit 101 that performs scanning along the rows (in the horizontal direction) by feeding, for one row after another, a signal φHm to the gate of a MOS transistor Td whose drain is connected to one of a plurality of horizontal scanning lines 104.
In this solid-state image-sensing device, each pixel, i.e., a unit pixel, is composed of a photodiode PD functioning as a photoelectric conversion element, an N-channel MOS transistor Ta having its gate connected to the cathode of the photodiode PD, an N-channel MOS transistor Tb having its source connected to the cathode of the photodiode PD, and an N-channel MOS transistor Tc having its drain connected to the source of the MOS transistor Ta. That is, the solid-state image-sensing device shown in FIG. 14 has a plurality of pixels each composed of MOS transistors Ta, Tb, and Tc and a photodiode PD. It is to be noted that, in FIG. 14, for simplicity's sake, only one pixel, in the nth row and mth column, is shown.
In this solid-state image-sensing device, the anode of the photodiode PD is grounded, and a direct-current voltage VDD is applied to the drains of the MOS transistors Ta and Tb. The gates of the MOS transistors Tb and Tc are connected to the reset line 102 and the vertical selection line 103, respectively. The source of the MOS transistor Td is connected to a horizontal signal line 105, so that an image signal is fed out by way of the horizontal signal line 105 via an output terminal OUT.
In the solid-state image-sensing device configured as described above, first, a high level is fed, as the signal φVSn, to the vertical selection line 103, so that the MOS transistor Tc is turned on. This causes a voltage commensurate to the amount of light incident on the photodiode PD to appear at the gate of the MOS transistor Ta. This causes an amplified current commensurate to the voltage appearing at the gate of the MOS transistor Ta to flow through the MOS transistor Ta, then through the MOS transistor Tc, eventually into the horizontal scanning line 104. Then, a high-level pulse is fed, as the signal φHm, to the gate of the MOS transistor Td, so that the output current fed to the horizontal scanning line 104 is fed out by way of the horizontal signal line 105 via the output terminal OUT.
Subsequently, the signal φVSn is turned to a low level, and then a high level is fed, as the signal φVRn, to the reset line 102, so that the MOS transistor Tb is turned on. As a result, the direct-current voltage VDD is fed through the MOS transistor Tb to the node between the cathode of the photodiode PD and the gate of the MOS transistor Ta, achieving resetting.
In the conventional MOS-type solid-state image-sensing device where, as shown in FIG. 14, a unit pixel is provided with an amplifying function by being provided with a MOS transistor Ta for signal amplification, the unit pixel requires, in addition to a photodiode, three or more MOS transistors. Moreover, many conductor lines need to be connected to each unit pixel to perform the outputting of an image signal and resetting alternately and to supply it with a supply voltage. These factors impose limits to the improvement of the open-area ratio and the miniaturization of the pixels.